Onboard acm-workload https://github.com/stolostron/acm-workload to prow, so we can use "/lgtm" "/approve" to manage code merging.

This acm-workload repo is only used internally to store workloads for testing the footprint occupied by ACM agents. Previously, these workloads were placed in haoqing’s private repo https://github.com/haoqing0110/acm-workload. We plan to transfer this private repo into stolostron https://github.com/stolostron/acm-workload. The built image will not be published to the [backplane-pipeline](https://github.com/stolostron/backplane-pipeline).